My BRAUNER VM1 modification

[MS Vienna]

Seems that there are some different versions of the VM1 around...

It´s good to know that the circuit is basically a G7 as it seems to be a good one. And given that Dirk Brauner was heavily into tube circuits even before he started making mics I think the amp design is fine enough as it is.

To avoid confusion I changed the Topic´s title to "My" Brauner VM1 modification. So here some infos about this specific mic.
Serial number is 1073 (which doesn´t like that top end zing ;-)

Tube is Philips 5840 (picture stock condition)

Capsule is K67/K47-hybrid concept, manufactured by MBHO.

Workmanship is excellent, acoustical concept questionable IMHO. Essentially it combines the shortcomings of the K67 (top end exaggeration) and the K47 (highly inconsistent frequency response from omni to figure eight being very midrange dominated in the latter pattern).

Furthermore the capsule base is really less than ideal. It´s an injection molded ABS part,  a small dome surrounded by a "moat" with flat ground (the grille entry starts a good deal higher than the ground of the capsule base). None of the great classic mics have that "feature" as from an acoustical point of view it makes no sense. Plus the capsule´s position within the grille is very low.

Add the flat top grille and you get me seriously scratching my head...what an acoustical mess!

So my next step was to modify the capsule base with various custom made wedged rings of Plexiglas in different dimensions and to elevate the whole thing in the grille by using spacers. No surprise - sound improved. After thorough experimentation I decided to throw out the complete base assembly and make a new one from scratch.

That was a pencil sketch to lay out dimensions and angles/shape of the new Plexiglas base.

(don´t be confused by the illustration of capsule/holder)

More to follow - have to go back to work now.

 

[micaddict]

Great stuff (again)! 

 

[Gus]

transistor switches the pad look at the microphone metas there are some threads
U67 plate operating point with cathode bias so not a G7
just another triode out to a transformer microphone

 

[ln76d]

transistor switches the pad look at the microphone metas there are some threads
U67 plate operating point with cathode bias so not a G7
just another triode out to a transformer microphone

Yes i serached metas earlier

I was hoping for schematic for it ;D

Here's your post from 2004:

The pad is switched remote from the power supply.  The microphone body has a 5v reed relay close to the capsule.  The body has a transistor that drives the relay coil using the heater supply.  I think this was done as to not have a current spike in the multi wire microphone cable.

I did not send out the supply schematic.  The Important voltages are marked on the body schematic.  The stock supply has an adjustable 3 term heater supply and a 5 cap 4 resistor well filtered HV supply not regulated.

One could voltage double  120 VAC (120 to 12, 12 to 120 transformers)to a few RCs use three 75V 1 watt zeners for about 225V then use a resistor and filter cap to bring it to about 185VDC adjust the last resistor for the correct B+.

The pattern control voltage should be adjusted for the capsule you will use.  I did not see the point of showing the voltages because they were picked for the VM1 capsule.  You should calculate the resistors for the capsule you want to build with.

  the schematic was sent out kind of as a learning tool.  There is nothing really new and different: however if you get a chance check out the VM1 it sounds very good.  I think it is the capsule grill and good parts used to build it.

 

[MS Vienna]

Thanks for the info on the transistor, always wondered what that thing does in there.

So for for the Plexiglas capsule base: That is quite easy to do really if you can use a drill press, basic lathe and preferably a band saw.
First I drilled the holes into the Plexi block.

Then cut it out and sanded it roughly round

Then went to the lathe. Now very few lucky ones may have a stepped ring collet to hold the piece. A simple but convenient solution here is to apply douple stick tape to the chuck jaws and then use the tailstock to center the work and press it against the chuck. Works great.

With the domed profile cut

After fine sanding it can easily polished a high gloss, glass like surface. Just take care not to overheat the material during sanding/polishing. To remove remaining polishing compound it´s best to use an ultrasonic cleaner filled with warm water and some mild dishwashing detergent (alcohol would immediately crack the piece).
I also made a new capsule holder from some PVC scrap and bought a threaded rubber element with elasticity suiting the capsule´s weight.
Components of new capsule base assembly:

So after all the effort I didn´t wan´t to go with the stock capsule anymore and put in something decent

It´s a K49 and the two sides are matched remarkably well.
Eventually the shape of the Plexiglas base is nothing new at all. It turned out to be pretty similar to a U67/M269-base and of course the dome now starts just at the lower grille entry ( failed to re-invent the wheel in this regard though as someone already was that clever half a century ago).

The very thin braids that connect the capsule are not my own idea as well. I´ve  got that from a meanwhile retired guy of AKG´s manufacturing department who contributed that to the X-series C414´s (which have a fantastic shock isolation of the capsule thanks to it).

I compared the mic to an all original M49(with K49). Overall sound balance is very similar. However the M49 has a juicy sweetness to it and a charme that all AC701 mics I heard so far share. The (Ex-)Brauner is extremely clean and clear throughout it´s dynamic range and stays true even at quite massive SPL´s without any signs of saturation. But if you look for Mojo look somewhere else.
That might not be the end of the story but the "wrongness" that originally bothered me has gone. Quite a step up.

 

[DerEber]

Thanks!
I see many interesting Mic treats popping up.
Great work!

 

[ln76d]

I compared the mic to an all original M49(with K49). Overall sound balance is very similar. However the M49 has a juicy sweetness to it and a charme that all AC701 mics I heard so far share. The (Ex-)Brauner is extremely clean and clear throughout it´s dynamic range and stays true even at quite massive SPL´s without any signs of saturation. But if you look for Mojo look somewhere else.
That might not be the end of the story but the "wrongness" that originally bothered me has gone. Quite a step up.

Great work!

Of course that AC701 brings some "mojo", but i think that there's few other factors which affect specific sound signature.
Excluding transformer which is important, M49 have also filters etc.
Of course am not saying about copy m49 sound signature, but it's good  example.
With Brauner, if you want to get some mojo, i would start from high ohm resistors and use much more 100M-250M range instead 3G.  Set new value of cathode capacitor (EF86 version - 47uF - according schematic).
You could try to use 3pF-10pF capacitor from plate to grid1 (M49C - C4 8pF). Sometimes nice option is cap from plate to ground 200pF - 1nF (M49C - C6 600pF), usual for little LPF, but can be set higher than usable frequency range, still makes some phase differences.  Cap between front and back diaphragm also can be tricky! If it's 1nF as on schematic - it's not bad. But i would try lower value 10pF for example, or since you have equipment and great skills for mechanical work, you could think about little switch for "cardioid only" setup (also as in M49 - S2) which makes difference.

 

[micaddict]

Brauner also offers the VMX which is the VM1 with some character added. The difference is clearly audible.
The VMA combines the VM1 and VMX in one microphone.
Here's the VMX. Notice also that Dirk's "character mics" have round tops FWIW.
http://www.brauner-microphones.de/en/products/vmx/

And they all come in pure cardioid (cardioid only) versions, too.

 

[MS Vienna]

Actually I compared a VMX directly to my modded VM1. My mic yet had a stronger, more pleasant character probably due to the Neumann capsule (even though it´s much cleaner than the M49).
The VMX had what I didn´t like in my VM1 in the first place. It wasn´t perceived that way just by me. One of the studio´s engineers described the VMX´s sound as "cultivated artificialness" which is an interesting way to express it but kind of nails it  (we´re aware that this is grumbling on a very high level ) That VMX too had a MBHO capsule that saw almost the same acoustical compromises within the grille except the domed top grill and a mesh arrangement that looked basically the same as on the VM1-KHE (coarse outer mesh). 

Thanks for the link to the Brauner-website,  just watched and read for a while. Didn´t find any specific info about their selfmade capsules but I´d be very interested to try one.

With Brauner, if you want to get some mojo, i would start from high ohm resistors and use much more 100M-250M range instead 3G.  Set new value of cathode capacitor (EF86 version - 47uF - according schematic).

I´m not particularly looking to inject mojo into my VM1. I think the amp ´s stable cleanness and transparency is a great quality on it´s own and there already are many mics with a strong native charakter anyway.
But I´d like to experiment with those GOhm resistors - I have no  clue about circuitry mods though.
My mic has two 3GOhm and one 1GOhm resistors. 
Should I replace the 3G with 100M and leave the 1G alone?
I really need advice here.

I have tried pure cardioid mode of course. A bit more output and even cleaner sound with a tad more resolution.
For now I don´t want to perform any irreversible modifications so I can always return the mic to original condition just in case  I´m going to sell it sometime in the future.


btw I recently did the Plexiglas dome mod to one of my TLM103 including capsule height correction to U87-level, basically giving it U67-gille acoustics.  Met my expectations. If there´s interest I can do an extra thread about it.

 

[ln76d]

You can use all three 100M or use one 1G at backplate (where probably sits currently).
Anyway i would try all three 100M. 
If you notice any noise issue, change two filtering caps from 10nF to higher value.
Probably it wouldn't be a problem only overtake posts of other users

Despite "mojo" things, you should really try new cathode cap. It's worth and it's just one capacitor
I would start testing with bigger values, simply soldering cap in parallel to the existing 47uF, if PCB space from traces side allow.
Small steps by 10uF, 22uF, 33uF, 47uF. I think there's no need to go higher. If this wouldn't meet expectations, then i would try go lower. Soldering 10uF and then adding another 10uF and next 22uF.

Please, even don't ask about starting other threads!
I think that i'm not the only one here, which really like to see your work!!!!!

 

[micaddict]

MS Vienna wrote:

btw I recently did the Plexiglas dome mod to one of my TLM103 including capsule height correction to U87-level, basically giving it U67-gille acoustics.  Met my expectations. If there´s interest I can do an extra thread about it.

Please do! I have a U67 and had an early TLM103. While being made decades apart, the headbaskets (the grill part) looked absolutely identical; size, shape, mesh, finish, everything. But the height of the capsules was very different. I've always wondered about the impact on tone.

And regardless, just like In76d said, any thread from you is very welcome indeed.

 

[abbey road d enfer]

My mic has two 3GOhm and one 1GOhm resistors. 
Should I replace the 3G with 100M and leave the 1G alone?
I really need advice here.

Without an actual schemo, it's difficult to predict exactly what would result, but it is certain that the LF response will be closer to about 20-30 Hz, instead of the typical 2-3Hz, which may produce a cleaner output, particularly if there is ambient VLF noise, that can't be heard, but may intermodulate with the useful signal.
A side-effect is that the self-noise of the head amp increases by about 10 dB. That is not a problem in most cases of close-miking.
The use of Gigaohm resistors is dictated by the need to publish a good noise figure. When it doesn't pose a problem, it is quite possible to decrease the value of these resitors, with the benefit of cleaner output; then the mic becomes unsuitable for VLN applications.

 

[MS Vienna]

Thanks for the suggestions and help!

I replaced just the two 3G resistors with 100M (which actually measured 90M).
The difference is very interesting and I like it!
The transients are smoothed in a pleasant and musical way. Less precision but more body and still quite enough resolution. Now the Bosch cap pays! I´ll try cardioid only again, could be interesting now.

I didn´t notice any relevant change in noise in my case. Perhaps it can be measured but since it´s a recording mic I care for things that can be heard.
That´s why I don´t care for a theoretical frequency response that goes down to 2-3Hz. Practically no musical information is happening down there. But much more important: No directional capsule can deliver an useful signal down there anyway just because of the mere way how a pressure gradient capsule works!  What you get here is just unwanted infrasonic signals like structure born noise and/or noise due to slow air flow - those things are unfortunately picked up very well by a gradient capsule.

There´s another aspect. Some years ago I asked veteran mic development engineer Konrad Wolf why those very low value resistors were used in the M251/M250 mics (30MOhm with 6072, 8MOhm with AC701). One (obvious) reason was because Telefunken wanted the mics´ low end response to roll off early.  But he also mentioned that the capsule´s noise is shorted better by those low values. I found that interesting. But then it´s quite obvious. The capsule´s insulation requirement is lowered considerably that way, meaning far better resistance to moisture noise.  That means very high reliability especially in close up vocal work!
A center terminated capsule clearly benefits from this too.

Despite "mojo" things, you should really try new cathode cap. It's worth and it's just one capacitor
I would start testing with bigger values, simply soldering cap in parallel to the existing 47uF, if PCB space from traces side allow.
Small steps by 10uF, 22uF, 33uF, 47uF. I think there's no need to go higher. If this wouldn't meet expectations, then i would try go lower. Soldering 10uF and then adding another 10uF and next 22uF.

I haven´t tried  it yet. Space is very tight there and I don´t have different caps in that value range readily available.
I have no idea what to expect from changing the cap´s value. What can I expect?

 

[micaddict]

I'm think I"m going to archive stuff like this:

I replaced just the two 3G resistors with 100M (which actually measured 90M).
The difference is very interesting and I like it!
The transients are smoothed in a pleasant and musical way. Less precision but more body and still quite enough resolution. Now the Bosch cap pays! I´ll try cardioid only again, could be interesting now.

I didn´t notice any relevant change in noise in my case. Perhaps it can be measured but since it´s a recording mic I care for things that can be heard.
That´s why I don´t care for a theoretical frequency response that goes down to 2-3Hz. Practically no musical information is happening down there. But much more important: No directional capsule can deliver an useful signal down there anyway just because of the mere way how a pressure gradient capsule works!  What you get here is just unwanted infrasonic signals like structure born noise and/or noise due to slow air flow - those things are unfortunately picked up very well by a gradient capsule.

There´s another aspect. Some years ago I asked veteran mic development engineer Konrad Wolf why those very low value resistors were used in the M251/M250 mics (30MOhm with 6072, 8MOhm with AC701). One (obvious) reason was because Telefunken wanted the mics´ low end response to roll off early.  But he also mentioned that the capsule´s noise is shorted better by those low values. I found that interesting. But then it´s quite obvious. The capsule´s insulation requirement is lowered considerably that way, meaning far better resistance to moisture noise.  That means very high reliability especially in close up vocal work!
A center terminated capsule clearly benefits from this too.

 

[ln76d]

Thanks for the suggestions and help!

I replaced just the two 3G resistors with 100M (which actually measured 90M).
The difference is very interesting and I like it!
The transients are smoothed in a pleasant and musical way. Less precision but more body and still quite enough resolution. Now the Bosch cap pays! I´ll try cardioid only again, could be interesting now.

I noticed that many times with tube microphones
With lower impedance somehow it sound much more pleasant. With  Gohm range resistors somehow it sounds always worse to me , typically i would call it these days "modern", less character - in the middle way to China

You could also try another thing at this position (could you? am really courious  your opinion), cap between two diaphragms - 3,3pF - 10pF range instead 1nF (according to hand drawn schematic, but i wouldn't be surprised if it's 10nF).

I didn´t notice any relevant change in noise in my case. Perhaps it can be measured but since it´s a recording mic I care for things that can be heard.
That´s why I don´t care for a theoretical frequency response that goes down to 2-3Hz. Practically no musical information is happening down there. But much more important: No directional capsule can deliver an useful signal down there anyway just because of the mere way how a pressure gradient capsule works!  What you get here is just unwanted infrasonic signals like structure born noise and/or noise due to slow air flow - those things are unfortunately picked up very well by a gradient capsule.

I also never found any noise problem, usual am using 22nF and resistor values starting from 33M. No problem at all.

There´s another aspect. Some years ago I asked veteran mic development engineer Konrad Wolf why those very low value resistors were used in the M251/M250 mics (30MOhm with 6072, 8MOhm with AC701). One (obvious) reason was because Telefunken wanted the mics´ low end response to roll off early.  But he also mentioned that the capsule´s noise is shorted better by those low values. I found that interesting. But then it´s quite obvious. The capsule´s insulation requirement is lowered considerably that way, meaning far better resistance to moisture noise.  That means very high reliability especially in close up vocal work!
A center terminated capsule clearly benefits from this too.

Great info!

I haven´t tried  it yet. Space is very tight there and I don´t have different caps in that value range readily available.
I have no idea what to expect from changing the cap´s value. What can I expect?

With cathode cap you can adjust low end, but it works little bit different than output cap. In most of circuits on which i worked , it was easier to find "sweet spot" of low end especially with more pleasant low mids.
Let say that output cap is like the gain control pot on microphone preamp, then cathode cap is additional "trim" control pot

 

[abbey road d enfer]

I don´t care for a theoretical frequency response that goes down to 2-3Hz. Practically no musical information is happening down there.

I fully agree, but the reason why mic designers extend as much as possible the LF extension is because it reduces Johnson noise.

But much more important: No directional capsule can deliver an useful signal down there anyway just because of the mere way how a pressure gradient capsule works!  What you get here is just unwanted infrasonic signals like structure born noise and/or noise due to slow air flow - those things are unfortunately picked up very well by a gradient capsule.

You are contradicting yourself here; one one hand you say that gradient capsules cannot respond to VLF (which is somewhat true), on the other hand you say they capture the very low frequency noise of aircon. Choose your camp!

There´s another aspect. Some years ago I asked veteran mic development engineer Konrad Wolf why those very low value resistors were used in the M251/M250 mics (30MOhm with 6072, 8MOhm with AC701). One (obvious) reason was because Telefunken wanted the mics´ low end response to roll off early. 

I don't understand why they would have wanted to truncate the LF response of their mics. That's a -3dB LF point at 65 Hz for the 6072 and 250 Hz for the AC701!

But he also mentioned that the capsule´s noise is shorted better by those low values.

I'm sorry, but I have to disagree with this person - or maybe you haven't correctly transcribed his words. Counter-intuitive as it may be, with a parallel RC circuit (which is what the head amp sees), the lower the resistance, the higher the noise. Loading a 80pF capsule with 30 Meg produces a noise voltage of about 7uV, slightly superior to the 6072's noise, for a net increase in hiss of about 4 dB.

I found that interesting.

But false...

But then it´s quite obvious.

Not so much...

The capsule´s insulation requirement is lowered considerably that way, meaning far better resistance to moisture noise.  That means very high reliability especially in close up vocal work!
A center terminated capsule clearly benefits from this too.

That is indeed true, and would indicate that the AKG designers have targetted the M250/251 to close-miced vocals, where the mic self-noise is not an issue.

 

[MS Vienna]

Various capacitors I ordered as possible cathode cap replacement were in the mail today. Hopefully I´ll find the time to do thorough testing soon!



Some clarification needed here:

But much more important: No directional capsule can deliver an useful signal down there anyway just because of the mere way how a pressure gradient capsule works!  What you get here is just unwanted infrasonic signals like structure born noise and/or noise due to slow air flow - those things are unfortunately picked up very well by a gradient capsule.

You are contradicting yourself here; one one hand you say that gradient capsules cannot respond to VLF (which is somewhat true), on the other hand you say they capture the very low frequency noise of aircon. Choose your camp!

Easy: In my opinion unwanted infrasonic signals like structure born noise and/or noise due to slow air flow are generally not useful signals.

But he also mentioned that the capsule´s noise is shorted better by those low values.

I'm sorry, but I have to disagree with this person - or maybe you haven't correctly transcribed his words. Counter-intuitive as it may be, with a parallel RC circuit (which is what the head amp sees), the lower the resistance, the higher the noise. Loading a 80pF capsule with 30 Meg produces a noise voltage of about 7uV, slightly superior to the 6072's noise, for a net increase in hiss of about 4 dB.

Moisture noise!  He said “das Kapselrauschen wird besser kurzgeschlossen” in the context of lower resistor values (to quote the actual words). 
Anyway, thanks for your additional info!

You showed very well how easy words could be interpreted to the point of being regarded as false without knowing their specific background. Of course it´s  necessary to point that out in the interest of truth and getting the whole picture right. I probably should have been more diligent here.

To understand statements of old pioneer AKG guys correctly it is necessary to know where they come from, plus some in-house terminology might be different in some cases from what others use. So if you understand it in a different way than it´s actually meant you could get the impression of being told nonsense or even being lied to while the person you´re speaking with has nothing evil in mind at all. 

Background in this case: Moisture noise always was a major concern especially with the earlier condenser capsules. So if capsule noise is mentioned it usually refers to moisture noise. Eventually there even were patents that exclusively address that issue.  In case you get a "The capsule is noisy" from service department there´s no use to explain to the guy how capsule noise voltage depends on load. It simply would mean that your capsule has moisture noise issues (e.g. is dirty).  I hope that helps clarifying the original statement.

Retrospectively I think it probably wasn´t wise to mention this stuff in the first place.

 

[Gus]

found this
http://recordinghacks.com/microphones/Brauner/VM1-KHE

 

[abbey road d enfer]

But much more important: No directional capsule can deliver an useful signal down there anyway just because of the mere way how a pressure gradient capsule works!  What you get here is just unwanted infrasonic signals like structure born noise and/or noise due to slow air flow - those things are unfortunately picked up very well by a gradient capsule.

You are contradicting yourself here; one one hand you say that gradient capsules cannot respond to VLF (which is somewhat true), on the other hand you say they capture the very low frequency noise of aircon. Choose your camp!

Easy: In my opinion unwanted infrasonic signals like structure born noise and/or noise due to slow air flow are generally not useful signals.

My comment was addressed only to this part: "...infrasonic signals like structure born noise and/or noise due to slow air flow - those things are unfortunately picked up very well by a gradient capsule".  Gradient capsules, with their intrinsic LF roll-off are not the worst in that respect. Undoubtedly, omnis are even "better" at picking VLF noise. I certainly do agree that for most musical recordings, nothing of real interest happens below 30 Hz. OTOH Ambisonics adepts seem to insist on the benefits of capturing and reproducing down to the bottom of the audio spectrum.

But he also mentioned that the capsule´s noise is shorted better by those low values. I'm sorry, but I have to disagree with this person - or maybe you haven't correctly transcribed his words. Counter-intuitive as it may be, with a parallel RC circuit (which is what the head amp sees), the lower the resistance, the higher the noise. Loading a 80pF capsule with 30 Meg produces a noise voltage of about 7uV, slightly superior to the 6072's noise, for a net increase in hiss of about 4 dB.

Moisture noise!  He said “das Kapselrauschen wird besser kurzgeschlossen” in the context of lower resistor values (to quote the actual words). 

You showed very well how easy words could be interpreted to the point of being regarded as false without knowing their specific background. Of course it´s  necessary to point that out in the interest of truth and getting the whole picture right. I probably should have been more diligent here.

To understand statements of old pioneer AKG guys correctly it is necessary to know where they come from, plus some in-house terminology might be different in some cases from what others use. So if you understand it in a different way than it´s actually meant you could get the impression of being told nonsense or even being lied to while the person you´re speaking with has nothing evil in mind at all. 

Background in this case: Moisture noise always was a major concern especially with the earlier condenser capsules. So if capsule noise is mentioned it usually refers to moisture noise. Eventually there even were patents that exclusively address that issue.  In case you get a "The capsule is noisy" from service department there´s no use to explain to the guy how capsule noise voltage depends on load. It simply would mean that your capsule has moisture noise issues (e.g. is dirty).  I hope that helps clarifying the original statement.

That's interesting! Apparently, he was more concerned with service issues than with measured performance. It has to be viewed in the perspective of the era's technology, where the tubes were a limiting factor, both in terms of noise and in terms of grid current, so the use of Gigaohm resistors was neither valuable nor practical.

Retrospectively I think it probably wasn´t wise to mention this stuff in the first place.

On the contrary. I believe it's always interesting adding some historic POV to a subject. It gives an opportunity to recall the different sources of noise in a condenser mic:
fixed noises: particle agitation (acoustic resistance noise), RC Johnson noise, head amp noise
extraneous noises: moisture noise, RFI/EMI noise, airborne noise
Some of these require conflicting optimization.

 

[j.frad]

Vert nice thread, I've been wondering about the improvement margin on my vmx.
I love the cleanliness but the highs van be a bit harsh.
So can you tell how much difference the capsule mount does or did you mount the k49 right away?